USS Thresher

The Vessel

USS Thresher (SSN-593) was the lead ship of the Permit-class nuclear attack submarines of the United States Navy, the design that represented the second generation of American nuclear attack boats after the Skipjack class. She was laid down at the Portsmouth Naval Shipyard in New Hampshire on 28 May 1958 and commissioned on 3 August 1961 as the first production submarine of an entirely new design philosophy: a single sound-quieted hull around a more powerful reactor, deeper test depth, better sonar, and the new Mark 37 homing torpedo.

Her design test depth was 400 metres, a depth that placed her operating envelope below any previous American submarine. Her hull was built of HY-80 high-tensile steel, a new alloy that allowed thinner plating at the required pressure. Her reactor was the S5W pressurised water unit, a development of the Nautilus's original S2W; her Mk 113 torpedo fire control system was the first fully digital fire-control system ever installed in a submarine. She was, at commissioning, the most technologically advanced submarine in the world.

She was also a prototype. Many of her systems had never been tested at sea in production form. Her first year of service produced a long punch-list of minor and major failures, including the saltwater piping system, the main seawater pump bearings, the atmosphere control system, and the sonar dome. In May 1962 she was withdrawn from the fleet for a 10-month availability at Portsmouth Naval Shipyard to address some 900 work items. The availability extended from May 1962 to April 1963.

The Voyage

She departed Portsmouth on 9 April 1963 under the command of Lieutenant Commander John Wesley Harvey, 35 years old, on her post-availability sea trials. Harvey had taken command during the availability; he was her second commanding officer. Her complement was 129: 16 officers, 96 enlisted, 17 civilian technicians from Portsmouth Naval Shipyard who had participated in the availability and were aboard to observe her sea-trial performance.

Her sea trials were to be conducted in a deep-water exercise area 220 miles east of Cape Cod, in waters of approximately 2,600 metres depth. The schedule called for three days of shallow-water testing followed by a deep dive to test depth on the fourth day. The accompanying submarine rescue ship was USS Skylark, which would remain on the surface as communications liaison. The submarine-rescue capability of Skylark was limited to the 260-metre surface-supplied diving range; she was irrelevant as a practical rescue asset for a submarine at test depth.

On the morning of 10 April 1963, Thresher began her deep-dive sequence. She reported to Skylark at successive depth intervals through the morning watch. At 09:13 Harvey reported Thresher experiencing "minor problems" and attempting to blow main ballast. The transmission was garbled on Skylark's receivers. At 09:17 a final broken transmission was received including the phrase "exceeding test depth". At 09:18, Skylark's hydrophones recorded the sound of a submarine imploding at depth. The implosion occurred at approximately 730 metres.

The Disaster

The implosion of a submarine at depth is functionally instantaneous. At 730 metres the seawater pressure is roughly 74 atmospheres; the collapse of a submarine hull under such pressure is complete in approximately 100 milliseconds. The crew, including the 17 civilian observers, died without any meaningful interval of awareness.

The Navy's investigation identified the proximate cause as the failure of a silver-brazed joint in the saltwater piping system at or near test depth. Salt water entered an engine-room space, shorted an electrical panel, and initiated an emergency reactor scram. The reactor scram removed main propulsion. The emergency main ballast blow failed to complete because moisture had clogged the air-drying strainers of the emergency blow system. Thresher sank from test depth to implosion without the means to arrest her descent.

The subsequent Naval Court of Inquiry and the follow-on DSRV submarine-rescue programme produced the most consequential reform in American submarine construction history. The SUBSAFE programme, established 20 December 1963 by direction of the Chief of Naval Operations, mandated extensive redesign of every component of every American submarine whose failure could result in flooding at depth. Silver-brazed joints of the type that had failed on Thresher were to be replaced by welded joints throughout the fleet. Every new American submarine since 1963 has been certified under SUBSAFE.

The United States Navy has lost no submarine to a SUBSAFE-certified hull integrity failure since Thresher. Two American submarines have been lost since 1963 (USS Scorpion, 1968), but Scorpion was not SUBSAFE-certified: she had been commissioned before the programme took effect. Every American nuclear submarine commissioned since January 1964 has been SUBSAFE-certified; none have been lost.

The Legacy

The Thresher wreck site was located by the bathyscaphe Trieste on a series of dives in June 1963, 350 kilometres east of Cape Cod at a depth of 2,560 metres. The hull had broken into six large sections distributed across an 800-metre debris field. The largest single piece, the stern, was approximately 20 metres long and lay upright on the Atlantic abyssal plain. The wreck has been visited by U.S. Navy deep-submergence vehicles on approximately a dozen subsequent occasions; the site is a protected war grave and classified as a controlled site under U.S. Navy regulations.

The 129 dead were memorialised on 12 September 1964 at Arlington National Cemetery with a bronze plaque bearing their names; the memorial is in Section 64 of the cemetery. The names include the seventeen Portsmouth Naval Shipyard civilians who had died alongside the uniformed crew. The shipyard itself, at the edge of the Piscataqua River where Thresher had been built, maintains a second memorial at the entrance to its Submarine Base.

The Permit-class was completed despite the loss of her lead ship; 13 more Permit-class boats were built through 1968, all to revised SUBSAFE standards. The last of them, USS Haddo (SSN-604), was decommissioned in 1991. The class had an excellent subsequent service record; none was lost.

The United States Navy declassified the 1963 Court of Inquiry report in 2020, along with portions of the saltwater piping analysis that had been held under classification for 57 years. The investigation's findings, once publicly readable, confirmed what the Navy had communicated privately to the families in the years after: the ship had been lost to the failure of an identifiable component, the failure mode had been engineered out of every subsequent American submarine, and the 129 dead had not died in vain. Every American submariner now at sea is operating under the rules that the Thresher's loss wrote.